1. Field of the Invention
The present invention relates to an improved antenna system for an automobile, which can efficiently detect radio or other waves received by the vehicle body and then transmit the detected signals to various built-in receivers in the vehicle body.
2. Description of the Prior Art
Antenna systems are essential for modern automobiles to positively receive external waves such as radio waves, TV waves, car-telephone waves and others at built-in receivers in the vehicle bodies. Antenna systems also are very important for citizen band tranceivers which are used to effect the transmission and reception of waves between an automobile and other stations.
A pole type antenna is generally known which projects outwardly from the vehicle body. Although such a pole type antenna exhibits favorable reception performance, it always interferes with the design and aesthetics of the automobile.
Moreover, the pole type antenna is subject to being damaged or stolen and also produces an unpleasant noise when an automobile on which the pole antenna is mounted runs at high speeds. It was frequently desired to eliminate the pole type antenna from the vehicle body.
Recently, frequency bands of radio or other waves to be received at automobiles are being increased. Thus, an automobile requires a plurality of antennas for receiving radio or other waves belonging to various frequency bands. It is undesirable for the number of antennas to be increased, because this will severely damage the aesthetics of the automobile and provide electrical interference between the multiple antennas which degrades reception performance.
Some efforts have been made to eliminate or conceal the pole type antenna. One of these efforts is that an antenna wire is applied, for example, to the rear window glass of an automobile.
Another effort is that surface currents induced on the vehicle body by radio or other waves are detected. This appears to provide the most positive and efficient antenna means. However, experiments showed that such antenna means provided no expected results.
One of the reasons why surface currents induced on the vehicle body by radio or other waves could not efficiently be utilized is that the level of such surface currents is not as high as expected. The prior art mainly utilized surface currents induced on the roof panel of the vehicle body, notwithstanding, one could not obtain a sufficient level of detected signals to be utilized.
The second reason is that surface currents include a very large proportion of noise. Such noise mainly results from the operation of ignition and regulator systems in an engine and therefore cannot be eliminated unless the engine is de-energized.
Under such disadvantageous circumstances, some proposals have been made to overcome the above problems in the prior art. Japanese Patent Publication No. Sho 53-22418 discloses an automobile antenna system utilizing currents induced on the vehicle body by radio or other waves. This antenna system comprises electrical insulation formed on the vehicle body at a location in which induced currents flow concentrically. The antenna system also comprises a sensor for directly detecting currents between the opposite ends of the electrical insulation. It is true that the antenna system can detect practicable signals being superior in S/N ratio. However, this antenna system requires a pickup device which must be installed in a notch formed on the vehicle body. This is not suitable for use in mass-production.
Another proposal is disclosed in Japanese Utility Model Publication No. Sho 53-34826 in which a pickup coil is mounted on the vehicle body at one of its pillars so as to detect current is flowing on the pillar. However, the pickup coil must be mounted on the pillar perpendicular to its length. This is not practical and also appears to be merely theoretical, since the pickup coil can provide no practicable output.